Electrical power supplies

ABSTRACT

A switched-mode power supply having a main output (O/P1) and one or more other outputs (O/P2, O/P3 and O/P4), wherein the outputs are regulated both by flyback control and forward control and wherein the power supply includes a transformer (T) having a primary winding, a main output winding (L2) and one or more other output windings (L3, L4 and L5), a primary converter (1) associated with the primary winding, a forward conversion circuit (S1, T, D2, D3, L H  and C1) associated with the main output winding (L2) for supplying the main output (O/P1), one or more flyback conversion circuits associated with the other output winding or windings (L3, L4 and L5) for supplying the other output or outputs (O/P2, O/P3 and O/P4), a forward control circuit (4) connected to the forward conversion circuit and arranged to control the main output (O/P1) in response to a signal derived from one or more of the other outputs (O/P2, O/P3 and O/P4), and a flyback control circuit (2) connected to the primary converter (1) and arranged to control the other output or outputs (O/P2, O/P3 and O/P4) in response to a signal derived from the main output (O/P1). In an alternative arrangement, a further main output winding (L1) is provided in a flyback conversion circuit arranged to control the forward control circuit (4).

This invention relates to electrical power supplies and is concerned, more particularly, with power supplies of the switched-mode converter type when provision must be made for more than one output.

In such power supplies, the existence of more than one output presents considerable problems if it is desired to regulate the outputs effectively and efficiently without undue complication and expense. Essentially, such power supplies at present available represent a compromise between these conflicting requirements and are designed having regard to the intended particular use of the power supply in terms of power range, degree of regulation required on each output, and so forth.

It is an object of the present invention to provide an improved general purpose switched-mode power supply having a number of outputs in which efficient regulation is achieved without undue complexity.

Some known switched-mode converters operate by using forward conversion in which the primary is coupled to the secondary through a transformer by normal transformer action i.e. energy demanded by the secondary is reflected to the primary and supplied by the primary instantaneously.

In other converters, which use flyback conversion, energy is stored in the primary of the transformer and when the primary switch is opened, the secondary voltages reverse and the stored energy is then transferred into the secondary circuits. In effect the device is not a transformer, but a coupled inductor, which provides the desired primary--secondary isolation. The chief merit is that no further inductors are required in the secondary.

The present invention involves in a switched-mode power supply having a main output and one or more other outputs, wherein the outputs are regulated both by flyback control, i.e. controlling the stored energy of flyback conversion, and forward control, i.e. controlling the direct energy of forward conversion.

In one arrangement, the main output is supplied by forward conversion controlled by a weighted average signal derived from one or more of the other outputs and the other outputs are supplied by flyback conversion controlled by the main output.

In a further arrangement, the flyback and forward conversions are both controlled by the main output.

In the accompanying drawings:

FIG. 1 is a block diagram of one form of power supply according to the present invention.

FIG. 2 is a block diagram of an alternative form of power supply according to the present invention.

FIG. 3 is a simplified circuit diagram of the power supply of FIG. 1, and

FIG. 4 shows waveforms associated with the power supply of FIG. 3.

In carrying the invention into effect according to one convenient mode by way of example, FIG. 1 is a block diagram of a switched-mode power supply having a primary converter 1 connected to a transformer T having windings L1 and L2 supplying a main output O/P1 and other, auxiliary windings L3, L4 and L5 supplying other outputs O/P2, O/P3 and O/P4. Winding L2 provides most of the power at the output O/P1 by forward conversion, whilst winding L1 provides some power by flyback conversion. The other outputs O/P2, O/P3 and O/P4 associated with windings L3, L4 and L5 are supplied with stored energy of flyback conversion controlled by a flyback control circuit 2 receiving a signal from the main output.

The main output O/P1 is under forward control by a weighted average signal derived by a forward control circuit 3 receiving inputs from the other outputs associated with windings L3, L4 and L5.

FIG. 2 is a block diagram of an alternative switched-mode power supply in which, as before, a signal is fed back from the main output O/P1 to the flyback control circuit 2 to apply flyback control. However, in this circuit, forward control applied to the main output by forward control circuit 4 is controlled by the current in the flyback portion of the main output.

This circuit has the following advantages for a general purpose, mid-range power supply:

(i) the relative complexity of forward conversion is only needed for the main output;

(ia) the main output current may be reduced to zero without affecting the other outputs,

(ib) the main output voltage may be varied without affecting the other outputs,

(ii) the regulation of the other outputs is theoretically as good as that for any flyback system, but in practice is better since the converter mark-space ratio is subject to much less fluctuation thus maintaining the effects of leakage inductance in the transformer at a sensible constant level throughout the power range;

(iia) the average value of the other outputs may be adjusted independently of the main output.

(iii) output current ripple on the main output is better than for either flyback or forward control alone. If required for the other outputs then forward control could also be applied to these.

(iv) peak power capability of the main output is as good as forward control alone and for any other output having forward control applied to it.

(v) at low power the printed circuit are efficiency is as good as flyback control alone, and at high power the printed circuit area efficiency for outputs having a forward conversion component is better than flyback control alone if not as good as forward control alone.

(vi) performance similar to separate forward and flyback convertors is achieved in a single converter employing one transformer, one set of primary switches and one primary control circuit.

The mode of operation is explained in more detail by reference to FIG. 3, a simplified circuit diagram of the power supply of FIG. 1, and FIG. 4 which includes waveforms associated with FIG. 3. In practice, switch S₁ is a semiconductor and S₂ may be semiconductor or magnetic.

Output O/P1 is produced by the combination of the outputs of secondary windings L1 and L2. With switch S₂ closed, winding L2 drives a conventional forward conversion circuit including switches S1a and S1b, transformer T, secondary diodes D2 and D3, inductor L_(H) and capacitor C1, which form a conventional transformer coupled "buck" converter. (Hence, the converter including elements L2, D2, D3, L_(H) and C1 may be considered as providing forward conversion.) When switch S₂ is open, this action is inhibited. If switch S₂ is then closed while switches S1a and S1b are closed, winding L₂, switch S₂, diodes D₂ and D₃, inductor L_(H) and capacitor C1 form a secondary "buck" regulator. Winding L1 and diode D₁ form a `flyback` connection which passes energy to capacitor C1 and ouput O/P1 while switches S1a and S1b are open, this energy having been stored magnetically in the transformer while switches S1a and S1b are closed. (Hence), primary converter 1 may be considered as providing flyback conversion.) Winding L1 serves to smooth the provision of energy to the output and hence may reduce the size of inductor L_(H) and/or capacitor C1. Winding L₁ may be omitted. Outputs O/P2, O/P3 and O/P4 are provided with flyback energy only, provided in a similar manner to that of output O/P1.

The flyback control circuit 2 controls the duty cycle of switches S1a and S1b to maintain output O/P1 at a constant voltage. The forward control average circuit 4 controls switch S₂ to maintain the average of outputs OP/2, OP/3 and O/P4 at a constant level. There may be any number (including one) of flyback type outputs.

FIGS. 4(a) and (b) are somewhat idealised voltage and current waveforms respectively appearing across the primary P of transformer T. The waveforms of FIGS. 4(a) and (b) result from the action of switches S1a and S1b which are respectively closed and opened during periods t₁ and t₂. During period t₂, the voltage across primary winding P is a reflected voltage from the secondary circuits. During period t₁, the voltage across winding P, V_(p), equals the impressed voltage V_(in). During period t₂, the value of V initially clamps to -V_(in) briefly and then to a lower level equal to the reflected voltage of the flyback outputs O/P2, O/P3 and O/P4.

The associated current waveform of FIG. 4(b) includes, during period t₁, a linear ramp of magnetising current which causes energy to be stored in transformer T. The stored energy is released to the flyback output during period t₂. Superimposed on this ramp after time delay t_(f) is a current pedestal that is the reflected current from output O/P1.

The dashed lines in FIG. 4(c) represent the voltage across winding L₂ as a replica, modified by turns ratio, of the voltage across winding P1. The application of voltage indicated by the dashed lines to output O/P1 is delayed by switch S₂ for a period t_(f), so that the voltage V_(f) at terminal F (FIG. 3) is as shown in full lines in FIG. 4(c). The corresponding current I_(f) at terminal F is shown in FIG. 4(d). In FIGS. 4(e) and (f) are shown for completeness, the waveforms of the voltage across the flyback outputs O/P2, O/P3 and O/P4 and the current through the associated windings L3, L4 and L5. While complete energy transfer is shown for illustration purposes, this is not a requirement of the invention.

During, during t_(f) (FIG. 4c)) (where t_(f) ≦t₁) switch S₂ is open. Therefore output O/P1 acts in a forward conversion mode only during the period (t₁ -t_(f)). Ignoring switching and resistive drop ##EQU1##

For V₁, the voltage at output O/P1=constant, any change in t_(f) must be accompanied by a corresponding change in t₁ which is accomplished by the flyback control loop and switches S₁.

However the value of t₁ determines the energy available for flyback outputs O/P2, O/P3 and O/P4. The value of t_(f) is varied via the forward control loop and switch S₂ to maintain the average voltages of the auxiliary flyback outputs constant. Note that any number of outputs may be averaged. For example, output O/P2 maybe regulated alone, in which case extremely good regulation is achieved at this output at the expense of degraded regulation on outputs O/P3 and O/P4, and so forth.

The net result of the two control loops is to allow independent adjustment of forward conversion and flyback conversion, effectively producing the performance of two separate converters, but with only one transformer and primary circuit.

The power supply shown in FIG. 2 is an alternative form which achieves a similar effect. The action of the switches and the feedback control circuit is the same as previously described. The forward control circuit 4 however senses the integral of the current flowing in winding L1 (whose presence is mandatory in this arrangement). This is a measure of the flyback energy being contributed to output O/P1 by winding L1. By maintaining this constant through the action of the forward control circuit 4 and switch S₂, and by proper selection of the turns on windings L1, L3, L4, etc., such that ##EQU2## is lower for output O/P1 than for the equivalent of any other flyback output, an improved regulation may be obtained from the flyback outputs. 

We claim:
 1. A switched-mode power supply having main load terminals and one or more auxiliary load terminals, wherein loads at the main and auxiliary load terminals are regulated both by controlling the stored energy of flyback conversion, and by controlling the direct energy of forward conversion, the power supply comprising a transformer having a primary winding, at least two main output windings and one or more other output windings, a primary converter associated with the primary winding, a forward conversion circuit associated with one of the main output windings for supplying the main load terminals, a flyback conversion circuit associated with the other of the main output windings for also supplying the main load terminals, one or more further flyback conversion circuits associated with the other output winding or windings for supplying the other load terminals, a forward control circuit connected to the forward conversion circuit for controlling the main load terminals in response to a signal derived from the flyback conversion circuit supplying the main load terminals, and a flyback control circuit connected to the primary converter and arranged to control the other load terminals in response to a signal derived from the main load terminals.
 2. A switched-mode power supply as claimed in claim 1 wherein N sets of auxiliary load terminals are included, where N is an integer greater than 1, and the forward control circuit includes means for averaging responses from a plurality of the N sets of auxiliary load terminals.
 3. A switched-mode power supply comprising a transfromer having: a primary winding and first and second output windings for respectively supplying a main load adapted to be connected to main output terminals and an auxiliary load adapted to be connected to auxiliary output terminals, a primary converter having variable on periods and off periods connected to the primary winding, a flyback control circuit connected to be responsive to the voltage across the main output terminals and connected to control the primary converter for controlling the load at the main output terminals by flyback conversion, the transformer being coupled to the auxiliary load via the second output winding, so that the primary converter during the off periods responds to a voltage of the auxiliary load as reflected to the transformer via the second output winding, and a forward control circuit connected to be responsive to the voltage reflected to the transformer via the second output winding for controlling the load at the main output terminals by forward conversion, whereby the load at the main output terminals is controlled by both flyback and forward conversion.
 4. A switched-mode supply according to claim 3, wherein the flyback control circuit is also arranged to control the second output winding by flyback conversion.
 5. A switched-mode power supply according to claim 3, wherein the forward control circuit is supplied by the second output winding.
 6. A switched-mode power supply according to claim 3, wherein the forward control circuit is supplied by the first output winding under flyback control.
 7. A switched-mode power supply according to claim 3, which includes N of the auxiliary output windings, where N is an integer greater than 1, and the forward control circuit is supplied by a plurality of the auxiliary output windings.
 8. A power supply circuit for supplying DC current to a pair of main load terminals and to a pair of auxiliary load terminals comprising a primary variable duty cycle switching converter for supplying variable duty cycle current pulses to a primary winding of a transformer including main secondary winding means for supplying current to the main load terminals and auxiliary secondary winding means for supplying current to the auxiliary load terminals, first means for converting AC to DC connected between the main secondary winding means and the main load terminals, second means for converting AC to DC connected between the auxiliary secondary winding means and the auxiliary load terminals, flyback control means responsive to the DC voltage at the main load terminals for controlling the duty cycle of the current supplied by the primary converter to the primary winding, the transformer storing energy while current is supplied to the transformer by the primary winding, the second AC to DC converting means being coupled via the auxiliary secondary winding means to the transformer so the stored energy is supplied to the auxiliary load terminals via the auxiliary secondary winding means while no substantial current is supplied to the transformer by the primary converter, and forward control means responsive to the value of a parameter resulting from current flowing in one of said converting means while no substantial current is supplied to the transformer by the primary converter for controlling the duration of current flow from the main secondary winding means to the first AC to DC converting means while current is supplied to the transformer by the primary converter.
 9. A power supply circuit according to claim 8, wherein the variable duty cycle current while flowing in the primary winding includes a ramping segment followed by a pedestal segment, the pedestal segment being reflected current from the main load terminals, and the forward control means includes switch means responsive to said parameter for coupling current from the main secondary winding means to the first AC to DC converting means only during the ramping and/or pedestal segments.
 10. A power supply circuit according to claim 8, wherein the duration controlling parameter is indicative of the integral of a current component supplied by the main secondary winding means to the first AC to DC converting means while no substantial current is supplied to the transformer by the primary converter.
 11. A power supply circuit according to claim 10, wherein the main secondary winding means includes first and second winding segments, the first converting means including: rectifier means connected between the first winding segment and the main load terminals for supplying DC voltage of a predetermined polarity to the main load terminals while no substantial current is supplied to the transformer by the primary converter; first and second diodes, inductor means and switch means connected between the second segment and the main load terminals so that (a) when the switch means is closed and while current is supplied to the transformer by the primary converter current flows in a first direction to the main load terminals from the second segment via the switch means, the first diode and the inductor means and (b) when the switch means is open current continues to flow in said first direction to the main load terminals from the inductor means via the second diode and negligible current flows from the second segment to the first diode, the first direction of current flow aiding the voltage of predetermined polarity supplied to the main load terminals, the duration of the second switch closing being responsive to the integral of current supplied by the main secondary winding means to the rectifier means.
 12. A power supply circuit according to claim 8, wherein the duration controlling parameter is indicative of the DC voltage at the auxiliary load terminals.
 13. A power supply circuit according to claim 12, wherein the main secondary winding means includes first and second winding segments, the first converting means including: rectifier means connected between the first winding segment and the main load terminals for supplying DC voltage of a predetermined polarity to the main load terminals while no substantial current is supplied to the transformer by the primary converter; first and second diodes, inductor means and switch means connected between the second segment and the main load terminals so that (a) when the switch means is closed and while current is supplied to the transformer by the primary converter current flows in a first direction to the main load terminals from the second segment via the switch means, the first diode and the inductor means and (b) when the switch means is open current continues to flow in said first direction to the main load terminals from the inductor means via the second diode and negligible current flows from the second segment to the first diode, the first direction of current flow aiding the voltage of predetermined polarity supplied to the main load terminals, the duration of the second switch closing being responsive to DC voltage at the auxiliary load terminals.
 14. A power supply circuit according to claim 8, wherein N pairs of auxiliary load terminals are provided, where N is an integer greater than 1, and the duration controlling parameter is indicative of the average value of the DC voltages at a plurality of said N pairs of auxiliary load terminals.
 15. A power supply circuit according to claim 14, wherein the main secondary winding means includes first and second winding segments, the first converting means including: rectifier means connected between the first winding segment and the main load terminals for supplying DC voltage of a predetermined polarity to the main load terminals while no substantial current is supplied to the transformer by the primary converter; first and second diodes, inductor means and switch means connected between the second segment and the main load terminals so that (a) when the switch means is closed and while current is supplied to the transformer by the primary converter current flows in a first direction to the main load terminals from the second segment via the switch means, the first diode and the inductor means and (b) when the switch means is open current continues to flow in said first direction to the main load terminals from the inductor means via the second diode and negligible current flows from the second segment to the first diode, the first direction of current flow aiding the voltage of predetermined polarity supplied to the main load terminals, the duration of the second switch closing being responsive to said average value of the DC voltages at the plurality of said N pairs of auxiliary load terminals.
 16. A power supply circuit according to claim 8, wherein the main secondary winding means includes first and second winding segments, the first converting means including: rectifier means connected between the first winding segment and the main load terminals for supplying DC voltage of a predetermined polarity to the main load terminals while no substantial current is supplied to the transformer by the primary converter; first and second diodes, inductor means and switch means connected between the second segment and the main load terminals so that (a) when the switch means is closed and while current is supplied to the transformer by the primary converter current flows in a first direction to the main load terminals from the second segment via the switch means, the first diode and the inductor means and (b) when the switch means is open current continues to flow in said first direction to the main load terminals from the inductor means via the second diode and negligible current flows from the second segment to the first diode, the first direction of current flow aiding the voltage of predetermined polarity supplied to the main load terminals, the duration of the second switch closing being responsive to said parameter.
 17. A power supply circuit according to claim 8, wherein the main secondary winding means includes a secondary winding, the first converting means includes first and second diodes, inductor means and switch means connected between the secondary winding and the main load terminals so that (a) when the switch means is closed and while current is supplied to the transformer by the primary winding current flows in a first direction to the main load terminals from the secondary winding via the switch means, the first diode and the inductor means and (b) when the switch means is open current continues to flow in said first direction to the main load terminals from the inductor means via the second diode and negligible current flows from the second segment to the first diode, and means for controlling the duration of closing of the switch means in response to said parameter. 